Brief Description of the Invention
The invention relates to a sewing machine, and more particularly relates to a bobbin thread tension adjusting device of a sewing machine which is operated in a timed relation with rotation of an upper drive shaft of the sewing machine to intermittently give a predetermined suitable bobbin thread tension during the stitching operation, thereby to produce proper stitches in dependence upon a kind of fabric to be sewn. For attaining this object, a bobbin thread tension lever is provided. The lever is normally biased in one direction by a spring to give a predetermined amount of pressure against the bobbin thread. A release device is operated in a timed relation with the upper drive shaft of the sewing machine to intermittently release the bobbin thread tension lever to an inoperative position during the stitching operation. A manually accessible device is operated to optionally adjust the bobbin thread tension.
According to conventional sewing machine, it has been possible to adjust tension of the upper thread by manipulating an external operating part such as a dial 40 as shown in FIGS. 6 and 7. It has, however, been impossible to adjust the tension of the bobbin or lower thread from the outside of the sewing machine. The tension adjustment of the bobbin thread has required the machine operator, for example, to take out a bobbin case, which is normally positioned in a loop taker, from the sewing machine, and then to adjust a tension of a spring (41) as shown in FIG. 6 which is provided on the bobbin case at the outlet thereof from which the bobbin thread is pulled out. Such an operation is very troublesome and time consuming for a machine operator. In any events, it has been difficult to adjust the bobbin thread from the outside of the sewing machine due to the structure of the loop taker, because the bobbin loaded with the lower thread is positioned in the loop taker and therefore the bobbin thread tension element has to be provided in the loop taker mechanism in such a manner that it may not hinder the passage of the upper thread loop around the loop taker as the loop is taken by the loop taker and enlarged therearound, and because it is also difficult to connect such a bobbin thread tension element to the external operating part.